This invention relates to a method for producing an optical fiber preform and fiber. More specifically, the method relates to efficiently producing optical fiber preforms and fibers having multiple segments therein.
Manufacturing of optical fiber preforms, i.e., the article from which optical fiber is drawn, is typically accomplished by methods such as Outside Vapor Deposition (OVD), Vapor Axial Deposition (VAD), Modified Chemical Vapor Deposition (MCVD) and Plasma Chemical Vapor Deposition (PCVD). In accordance with one method, a multi-segment profile in the preform (corresponding to a multi-segment profile in the optical fiber drawn therefrom) is formed by an OVD method. In the OVD method, silica-containing soot 22 is deposited onto a rotating and traversing mandrel 24 as indicated by arrows A and Axe2x80x2 of FIG. 2 to form a porous core soot preform 20. To form the soot 22, a glass precursor 28 is provided, preferably in gaseous form, to the flame 30 of a burner 26. The flame 30 is formed by combusting a fuel 32, such as methane, while providing a combustion supporting gas, such as oxygen 34. The core soot preform 20 may be up-doped with a dopant such as germania oxide, for example, to raise its refractive index. This may be accomplished, for example, by providing a glass precursor 28, such as SiCl4, to the burner 26 in gaseous form along with a gaseous dopant compound, such as GeCl4. The doped silica-containing soot preform 20 is then dried and consolidated in a consolidation furnace 29, such as shown in Prior Art FIGS. 3 and 4 to form a consolidated core blank 31. A helium and chlorine gas atmosphere, for example, in the consolidation furnace is used to dry the preform and remove water prior to vitrification into glass at a temperature of about 950xc2x0 C. to 1250xc2x0 C. Pure helium is generally provided during consolidation and the temperature is higher, for example, between about 1390xc2x0 C. to 1535xc2x0 C.
Following consolidation, next, as shown in FIG. 5, the consolidated core blank 31 is placed in a cane draw furnace 37 and is stretched into a length of core cane 33 from which multiple core cane segments 35 are derived. At the same time, the centerline aperture is closed by application of, for example, a vacuum. The draw tension and preform downfeed rates (indicated by arrow B) are controlled by suitable control method 38 to provide a core cane length 33 of preferably substantially constant, predetermined diameter do. The diameter do is controlled by feedback of a measured diameter signal from an appropriate non-contact sensor 39 to the control apparatus 38. In response, the controls 38 may adjust the tension applied at the tension apparatus 40 whereby lowering the tension raises the diameter do and raising it lowers the diameter do. At predetermined lengths, the cane is cut, such as by a flame cutter 42, to form a predetermined length core cane segment 35 (FIG. 6). This core cane 35 represents the first segment 44 of the final preform, as illustrated in FIG. 1.
The second preform segment 45, which is a down-doped moat, is formed by depositing on the core cane segment 35 additional silica-containing soot. This step looks identical to FIG. 2 except that the mandrel is now the previously made core cane 35. The soot deposited is preferably silica soot formed by providing the glass precursor 28 such as SiCl4 to the flame 30 and oxidizing the precursor to form SiO2. Next, the soot-laden core cane 41 is placed in a furnace 29 as is described in Berkey U.S. Pat. No. 4,629,485 and the soot, after being dried, is subjected to a fluorine-containing atmosphere. This dopes the soot with fluorine. Subsequently, the doped-soot preform 41 is consolidated, as shown in FIG. 7. Again, the resultant consolidated preform (now containing two core segments) is drawn into a core cane as is shown in FIG. 5. The only difference is that the consolidated preform now includes a core at its centerline, rather than a centerline aperture as shown in FIG. 5.
To make the third up-doped segment 46 (FIG. 1), the process of FIG. 2 is again repeated where a glass precursor 28 is provided to the flame 30. A desired amount of dopant compound, such as GeCl4, is also provided to achieve the profile preferably as shown in the third segment 46 of FIG. 1. This is accomplished by gradually turning on the supply of dopant compound 36 at the innermost part of the segment and gradually turning it off towards the outermost portion of the segment 46 by controlling the mass flow controllers V. Once the additional soot segment is formed, it is again dried and consolidated as shown in FIG. 7. Once consolidated, it is again drawn into a core cane segment as described with reference to FIG. 5. As should be recognized, the core cane 10 segment now contains three segments 44, 45 and 46 therewithin.
In the final step, the core cane segment is overclad with silica-containing soot by the method shown in FIG. 2 wherein the cladding preferably comprises essentially SiO2. Again, the soot preform is dried and consolidated as heretofore mentioned to form a fourth segment 48 and to form the final consolidated optical fiber preform. The resulting final consolidated preform 50 is then placed in a draw furnace 52 as shown in FIG. 8, heated and drawn into an optical fiber 54 in a helium gas atmosphere by conventional methods and apparatus. The fiber 54 is then cooled in cooling chamber 55 and measured for final diameter by non-contact sensor 56. One or more coatings are applied and cured by coating apparatus 58, as is also conventional. During draw, the fiber 54 passes through a tension assembly 60 whereby tension is applied to draw the fiber 54 from the preform 50. The tension is controlled via control apparatus 61 to maintain the fiber diameter at a predetermined set point. Finally, the coated fiber 54 is wound by feedhead 62 onto a fiber winding spool 64.
It should be readily apparent that the prior art, multi-step, OVD process is complex, and therefore time intensive. Moreover, because of the multiple steps involved to arrive at the final optical fiber preform, it is sometimes difficult to achieve consistent profiles. Further, it is also possible to experience high levels of scrap.
Thus, it should be apparent that there is a long felt and unmet need to produce optical fiber preforms cost effectively, efficiently and with greater control of the optical parameters and index profiles.
The manufacturing method in accordance with a first embodiment of the invention provides a multi-segment preform that may be produced in a highly efficient manner with improved profile predictability and possibly lessened scrap. The method of manufacturing a multi-segment optical fiber preform, comprises the steps of forming a core cane segment, which preferably has a germania dopant therein, providing a delta of between about 0.2%-3%, inserting the segment into a sleeve formed by and inside method such as MCVD or PCVD and then collapsing the sleeve onto the cane. Other suitable inside methods may alternatively be employed. Fiber may then be drawn therefrom by conventional methods. The result is a detailed refractive index profile that can be readily made with fewer steps than the prior art method.
In particular, it has been found that the ring shape can be manufactured advantageously with a great amount of precision and, in particular, latent rings (rings that are positioned some finite distance away from the outer edge of the moat) may be manufactured very precisely. Further, the refractive index profile may be made with better repeatability and possibly with a lesser amount of scrap. Advantageously, new refractive index profiles may be made in accordance with the present method whereby heretofore using prior art methods, glass crizzling at the segment interfaces has occurred. The core cane, in accordance with the invention is preferably formed by an OVD method wherein a core soot region is formed by depositing silica-containing soot onto an outside of a rotating deposition surface, the core soot region is then dried and consolidated in a consolidation furnace to form a consolidated core blank, followed by drawing from the consolidated core blank the core cane segment having an outer dimension do.
In accordance with the invention, the sleeve is formed by an IVD method, such as an MCVD or, more preferably, a PCVD process. The core cane segment is inserted into the sleeve, preferably purged with a purge gas, and the sleeve is collapsed thereon to form a core-sleeve assembly. The sleeve is preferably manufactured by forming doped-silica glass deposited onto the inside of a silica-containing glass tube by supplying glass precursor together with a dopant compound to the tube""s cavity. The sleeve is doped such that it preferably includes a down-doped inner radial segment preferably with a delta between xe2x88x920.1% and xe2x88x921.2% and an outer up-doped radial segment preferably with a delta between 0.1% and 1.2%, both preferably measured as compared to pure silica. However, it should be recognized that the tube may also include a refractive index altering dopant, such as fluorine. Most preferably, the down-doped radial portion comprises fluorine and is located on an inner portion of the sleeve. The up-doped portion comprises germania and is located at an outer portion of the sleeve (located radially outward from the inner portion).
In accordance with another embodiment of the invention, silica-containing cladding is then provided after stretching the core-sleeve assembly to form a multi-segment core cane. The cladding may be formed by OVD process or by a cladding tube that is inserted over and collapsed onto the multi-segment cane. The resultant multi-segment preform is formed by consolidating the cladding soot or collapsing the cladding tube onto the core cane. From this, optical fiber including many complex up-doped and down-doped refractive index profiles may be manufactured.
The invention herein results in a method whereby better controls on the individual segments are provided because each may be individually controlled for dimensional characteristics and refractive index characteristics thereby better controlling mode field diameter, effective area, dispersion, and attenuation. Further, the numbers of process steps are significantly reduced. Additionally, the amount of scrap may be reduced because out of tolerance segments may be individually screened and scrapped and, therefore, not be incorporated into the final product. This further enhances the capability of closely and precisely achieving a particular target profile. Further, it is believed that the centerline dip (a dip in the refractive index profile at the center of the core) experienced by preforms made by the prior art method may be reduced by the present invention. Additionally, it is believed that better control of the depth of the fluorine moat may be accomplished by the invention. Moreover, because of the lesser number of manufacturing steps, the number of glass/soot interfaces is reduced, thereby reducing the attenuation increases associated with such interfaces.
In accordance with another embodiment of the invention, a method of manufacturing a multi-segment optical fiber is provided comprising the steps of forming a core soot preform by depositing silica-containing soot onto an outside of a rotating deposition surface, consolidating the core soot preform in a consolidation furnace thereby forming a consolidated core blank, drawing from the consolidated core blank to form at least one core cane segment having an outer dimension do; forming a sleeve on an inside of a tube wherein the sleeve includes one or more down-doped radial portions and one or more up-doped radial portions, preferably as compared to silica, inserting the core cane segment into the sleeve, collapsing the sleeve around the core cane segment to form a core-sleeve assembly, drawing the core-sleeve assembly forming a multi-segmented core cane, cladding on an outside of the core-cane to form an optical fiber preform, and drawing the optical fiber from the optical fiber preform. It should be recognized that the one or more down-doped portions may include a moat and a gutter, for example. Further, the one or more up-doped portions may include multiple spaced rings.
According to another embodiment of the invention, a method of manufacturing a multi-segment optical fiber preform is provided. The method comprises the steps of forming a core cane including a first up-doped portion and a down-doped portion by an OVD process, forming a sleeve including a second up-doped portion by one of a MCVD and PCVD process, inserting the core cane into the sleeve, and collapsing the sleeve around the core cane to form a cane-sleeve assembly.
According to another embodiment, the method of manufacturing a multi-segment optical fiber preform comprising the steps of forming a core cane including a first up-doped portion and a down-doped portion, forming a sleeve on an inside of a tube including a second up-doped portion, inserting the core cane into the sleeve, and collapsing the sleeve around the core cane to form a cane-sleeve assembly.
Preferably, the aforementioned cane-sleeve assemblies are drawn into at least one core cane and additional cladding is formed on the outside of a segment thereof. The cladding may include deposited soot that is subsequently consolidated, or a cladding tube inserted over, and collapsed onto the multi-segment core cane. Fiber may then be draw from the assembly.
Other features and details of the present invention will be apparent from the appended specification, claims and drawings.